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Headlines Published on 27 March 2007

Title The giant that is actually a dwarf

The European Southern Observatory (ESO), a European intergovernmental organisation for astronomical research, has made a startling discovery, effectively knocking down a 23-year-old belief that the celestial couple, formed by the galaxies NGC 5011 B and C, was at the same distance from Earth. Using a 3.6-m ESO telescope at the La Silla Observatory in Chile, the astronomers soon realised that the galaxy NGC 5011C is not a giant but a dwarf galaxy situated in the Milky Way area. The results of this study are presented in an article for the Astronomical Journal, currently in press.

The two galaxies NGC 5011B (top) and NGC 5011C (bottom blue galaxy) © ESO
The two galaxies NGC 5011B (top) and NGC 5011C (bottom blue galaxy)

Astronomers say that because NGC 5011C lacks distinctive features and has a low density of stars, they may classify the galaxy as a nearby dwarf elliptical galaxy. But NGC 5011C, they add, is located in the direction of the Centaurus constellation, towards the Centaurus A group of galaxies and the Centaurus cluster of galaxies. This may potentially lead others to believe that it belongs to the same cluster that the lenticular galaxy NGC 5011B belongs to.

For ESO's Ivo Saviane, who collaborated with Helmut Jerjen from the Australia-based Mt Stromlo Observatory, a problem does exist. "Despite the small distance between the two galaxies, this would imply from their projection on the sky if they were indeed at the same distance only 45 000 light-years, half the size of our Milky Way there is no obvious sign of interaction between the two," Saviane explained. He added that if the two galaxies were at the same distance, the NGC 5011C would be bigger than NGC 5011B in real size. End result? NGC 5011C would be a galaxy never before seen.

The two astronomers used the ESO 3.6-m telescope to take images and spectra of the galaxies. Despite what previous literature says, the galaxies actually have different redshifts, with NGC 5011C moving away from us five times slower than NGC 5011B on the sky. "This indicates they are at different distances and not at all associated," Jerjen said. "Clearly, NGC 5011C belongs to the close group of galaxies centred around Centaurus A, while NGC 5011B is part of the much further Centaurus cluster."

The two galaxies also have diverse central properties. NGC 5011C seemingly contains about 10 million times the mass of the Sun in stars, thus making it a true dwarf galaxy, while NGC 5011B has more heavy chemical elements than NGC 5011C. The astronomers also found that NGC 5011C lies 500 000 light-years away from the dominant galaxy in its group, Centaurus A (NGC 5128).

More information:

  • European Southern Observatory
  • Cornell University Library of e-prints

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